Field of the Invention
The present invention relates to a method for producing a molded article of a fiber-reinforced plastic, comprising molding a fiber-resin mixture (composite material) containing an inorganic reinforcing fiber and a thermosetting resin under heat and pressure.
Description of the Related Art
A molded article of a fiber-reinforced plastic containing an inorganic reinforcing fiber and a thermosetting resin has a light weight and an excellent mechanical property such as strength or stiffness, and thus has been widely used in various fields. The molded fiber-reinforced plastic article can be produced via a prepreg, which is prepared by impregnating the inorganic reinforcing fiber with the thermosetting resin. The prepreg is attached to a mold and formed into a desired shape under heat and pressure by using an autoclave, a press, etc. Then, the thermosetting resin is hardened in this state to obtain the molded fiber-reinforced plastic article.
Examples of the inorganic reinforcing fibers include carbon fibers prepared by carbonizing a material such as an acrylic fiber or a pitch fiber at a high temperature, glass fibers prepared by melting an alkali-free glass such as a silica glass and by drawing the molten glass into a fibrous form, and ceramic fibers mainly composed of an alumina and a silica.
To produce the molded fiber-reinforced plastic article, the prepreg (prepared by impregnating the inorganic reinforcing fiber with the thermosetting resin) may be cut to a horizontal and vertical size of 150 mm or less, and the cut prepreg sheets may be randomly stacked. Alternatively, the thermosetting resin may be filled with a short fiber of the inorganic reinforcing fiber, or the thermosetting resin may be cast (injected) into a base material of the inorganic reinforcing fiber (particularly a woven fabric, a mat material, etc.) without impregnating the inorganic reinforcing fiber with the thermosetting resin.
Incidentally, to produce the molded fiber-reinforced plastic article with a desired mechanical property suitable for intended application, the volume ratio of the inorganic reinforcing fiber to the entire molded article (the fiber volume content Vf) has to be equal to a predetermined value. The fiber volume content Vf is selected depending on the orientation, weave, and filling form of the inorganic reinforcing fiber, the types of the inorganic reinforcing fiber and the matrix resin, the intended application of the molded article, or the like.
Therefore, when the actual fiber volume content Vf of the molded fiber-reinforced plastic article obtained by the above production process (hereinafter referred to also as the actual measured value) is different from the predetermined fiber volume content Vf (hereinafter referred to also as the predetermined value), the desired mechanical property cannot be achieved in some cases. For example, when the actual measured value is smaller than the predetermined value, the resultant molded fiber-reinforced plastic article may be insufficient in strength or stiffness. On the other hand, when the actual measured value is larger than the predetermined value, the molded fiber-reinforced plastic article may be insufficient in toughness. Thus, in either event, the molded fiber-reinforced plastic article may fail to have excellent mechanical property. Consequently, to produce the molded fiber-reinforced plastic article with the fiber volume content Vf equal to the predetermined value, it is necessary to control the process of molding a mixture containing the inorganic reinforcing fiber and the thermosetting resin (hereinafter referred to also as the fiber-resin mixture) under heat and pressure.
For example, in view of producing the molded fiber-reinforced plastic article with the controlled fiber volume content Vf, a method for preventing leakage of a resin from a fiber bundle in the process of heat-hardening a fiber-resin mixture is proposed in Japanese Laid-Open Patent Publication No. 2008-296494.
Specifically, a fiber bundle is impregnated with a first resin by a resin impregnation apparatus. Then, the first resin and the fiber bundle are covered with a second resin by a resin coating apparatus. The second resin has a viscosity higher than that of the first resin, and the second resin is hardened more rapidly than the first resin in the process of heat-hardening the fiber-resin mixture. As a result, the leakage of the first resin can be prevented, and the fiber volume content Vf can be controlled.